Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  R' : A → B → Prop
  x : A
  y : B
  l : list A
  l' : list B
  H : R x y
  H0 : Forall2 R l l'
  IHForall2 : Forall2 R' l l'
              → Forall2 (λ (x : A) (y : B), R x y ∧ R' x y) l l'
  H2 : R' x y
  H3 : Forall2 R' l l'
  ============================
   (R x y ∧ R' x y)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := x
of type
intropattern
id0 := R
of type
intropattern
flags := 
of type
tauto_flags
X2 := B → Prop
of type
constr of type tacvalue